Micaschist

Mica-schist, a rock composed of quartz and mica foliated. The mica being in thin sheets causes the rock to split readily or be schistose. The mica is usually muscovite, but sometimes biotite. Garnet is a common accessory mineral; tourmaline and felspar less so. By addition of felspar it merges into gneiss. It sometimes appears to show traces of current-bedding and other indications of being originally sedimentary. It occurs in round granite bosses as a metamorphic zone, a mile or so broad, shading into slate or greywacke, and evidently due to contact-metamorphism; but it also forms vast regions in Norway, Scotland, the Alps, and elsewhere, of Archaean age, but not so clearly metamorphic in origin.